1. Technical Field of the Invention
The present invention relates generally to tire condition monitoring systems for vehicles. More particularly, the invention relates to a direct-type tire inflation pressure sensing apparatus for a vehicle which includes command signal receivers that have a variable receiver sensitivity.
2. Description of the Related Art
Conventional direct-type tire inflation pressure sensing apparatuses for vehicles are generally configured with at least one transmitter and a receiver.
The transmitter is directly installed to a wheel of the vehicle and includes a pressure sensor working to sense the inflation pressure of a tire mounted on the wheel. The transmitter is configured to transmit a pressure signal that indicates the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes an antenna. The receiver is configured to receive the pressure signal transmitted from the transmitter via the antenna and determine the inflation pressure of the tire based on the pressure signal.
In such direct-type tire inflation pressure sensing apparatuses, the transmitter may be configured to further include a command signal receiver or a command signal receiving circuit (For example, as disclosed in European Patent No. 1197356).
The command signal receiver is usually in a sleep mode. However, when an electromagnetic wave is transmitted to the command signal receiver, it “wakes up” to receive the electromagnetic wave. Then, the transmitter determines whether the electromagnetic wave is a command signal that indicates a predefined instruction for the transmitter or merely a noise from the external environment. If the electromagnetic wave is a command signal, the transmitter performs a predetermined task (e.g. transmitting the pressure signal) according to the instruction indicated by the command signal, and then the command signal receiver returns to the sleep mode. Otherwise, if the electromagnetic wave is merely a noise, the command signal receiver immediately returns to the sleep mode so as to suppress the electric power consumption (i.e., the standby current) thereof.
However, with such a configuration, when the vehicle is parked in an area where there exist electromagnetic wave noises with high levels, those noises will be applied to the command signal receiver of the transmitter. Consequently, the command signal receiver will repeat “waking up” and “going to sleep” for a long time, thus increasing the electric power consumption of the transmitter. Accordingly, in the case of the transmitter being supplied with electric power by a battery, the service life of the battery will be reduced.
As a solution for the above problem, one may consider lowering the receiver sensitivity of the command signal receiver so as to prevent the command signal receiver from being activated by “fake” command signals (i.e., noises). However, this will, at the same time, increase the necessary level (or strength) of “true” command signals for activating the command signal receiver. Consequently, in some cases, an intended transmission of a command signal to the command signal receiver could not cause the transmitter to perform a necessary task.